In the graphic arts industries and the like, a photosensitive material to make an original transparency signify photosensitive materials which are used for converting continuous-tone images of originals into half-tone images or for photographing line originals or the like, which is involved in a photomechanical process.
In the usual production of printing plates utilizing such photosensitive materials as described above, partial or overall minute retouch of image is carried out. This is done with the intention of reproducing the delicate tones of originals so that they have excellent printing characteristics or satisfying the artistic expression of the printed image. This is frequently carried out by subjecting these materials to a processing referred to as a reduction processing. This involves reducing the dot area of half-tone images, or increasing or decreasing the width of line images, or so on.
Accordingly, whether or not the photosensitive material have an aptitude for a reduction processing has become very important of such materials.
Reduction processing of the photosensitive material having dot images of line images formed through exposure and subsequent development steps is achieved by bringing metallic silver, by which the dot or line images are formed, into contact with a reducing solution. Various kinds of reducing solutions are known, specific examples include those containing as a reducing component a permanganate, a ferric salt, a ceric salt, a hexacyanoferrate (III), a dichromate, a persulfate or so on are described in, for example, C. E. K. Mees, The Theory of the Photographic Process, pp. 783-739, Macmillan, New York (1954).
However, since the reduction processing consists in, in the ultimate analysis, oxidation of a silver image and dissolution of the resulting oxidized silver, a decrease in dot area of a half-tone image due to the reduction processing is accompanied with a decrease in dot density. Therefore, the range of the retouching possibilities with reduction processing is reduced depending on the degree of the decrease in per dot density which the reduction processing causes along with the decrease in dot area. That is to say, a measure of the range of the retouching possibility relating to a half-tone image can be represented by the greatest possible extent that a dot area of the half-tone image can be reduced to as the per dot density is kept at some definite value or above.
In this specification, the magnitude of decrease of the dot area after receiving the reduction processing compared to the dot area before receiving the processing, when the dot density is reduced by the processing to the lowest possible value that the dot density must have in the photomechanical process, is expressed in terms of "reduction width". One aptitude for reduction processing increases as the reduction width increases.
As an example of a technique for enhancing the aptitude for reduction processing, there has been known the method described in published unexamined Japanese Patent Application No. 68419/77 wherein the reduction processing is carried out in the presence of specified mercapto compounds. However, such a method requires the use of special reducing solutions, which are difficult to use due to differences in reduction speed and so on from conventionally used ones.
The covering power of a gelatin-silver halide emulsion can be increased by adding thereto a natural or a synthetic macromolecular compound other than gelatin, such as poly-N-vinyllactams, starch, dextrin, laminarin, mannanhydroxyethylated cellulose, or the like. This is disclosed in, for example, U.S. Pat. Nos. 3,058,826 and 3,003,878; French Pat. No. 1,261,846; Belgian Pat. Nos. 611,622 and 611,623. Although it is possible to extend the reduction width through enhancement of the covering power attained in the above-described manner, the emulsion film becomes too soft. Thus the film can not obtain the mechanical strength necessary if the above-described additives having high molecular weights are used in such amounts as to improve upon the covering power.
The most effective measure for improving upon the aptitude for reduction processing through expansion of the reduction width is increasing the amount of silver to be used for the formation of image. This is because the reduction processing, as described hereinbefore, consists in dissolution of silver image through oxidation thereof. Therefore, the possibility of effecting retouching of the image by reduction processing generally increases along with the per unit area amount of image-forming silver.
Accordingly, the reduction width becomes wider as the per unit area coverage of silver halide used in the process photosensitive material is increased. However, as is universally known, silver is a very expensive and precious metal and therefore, an increase in the coverage of silver is undesirable with respect to saving money and resources.
Accordingly, production of a photosensitive material to make an original transparency having the desired characteristics with the use of the least possible amount of silver has been an important subject in this art.